Apply by doing:
       cd /usr/src
       patch -p0 < 015_aty2k.patch
       cd usr.bin/at
       make
       make install

Index: usr.bin/at/at.c
===================================================================
RCS file: /cvs/src/usr.bin/at/at.c,v
retrieving revision 1.16
retrieving revision 1.17
diff -u -r1.16 -r1.17
--- usr.bin/at/at.c     1998/07/09 20:40:58     1.16
+++ usr.bin/at/at.c     1999/12/15 05:33:06     1.17
@@ -197,7 +197,7 @@

       sigaction(SIGINT, &act, NULL);

-       (void)strcpy(atfile, _PATH_ATJOBS);
+       (void)strlcpy(atfile, _PATH_ATJOBS, sizeof atfile);
       ppos = atfile + strlen(atfile);

       /*
Index: usr.bin/at/parsetime.c
===================================================================
RCS file: /cvs/src/usr.bin/at/parsetime.c,v
retrieving revision 1.8
retrieving revision 1.9
diff -u -r1.8 -r1.9
--- usr.bin/at/parsetime.c      1999/03/21 04:04:42     1.8
+++ usr.bin/at/parsetime.c      2000/01/05 08:06:25     1.9
@@ -443,21 +443,23 @@
       struct tm *tm;
       int mday, mon, year;
{
-       if (year > 99) {
-           if (year >= TM_YEAR_BASE)
-                   year -= TM_YEAR_BASE;
-           else
-                   panic("garbled time");
-       } else if (year != -1) {
-               /*
-                * check if the specified year is in the next century.
-                * allow for one year of user error as many people will
-                * enter n - 1 at the start of year n.
-                */
-               if (year < tm->tm_year % 100 - 1)
-                       year += 100;
-               /* adjust for the year 2000 and beyond */
-               year += tm->tm_year - (tm->tm_year % 100);
+
+       /*
+        * Convert year into tm_year format (year - 1900).
+        * We may be given the year in 2 digit, 4 digit, or tm_year format.
+        */
+       if (year != -1) {
+               if (year >= TM_YEAR_BASE)
+                       year -= TM_YEAR_BASE;   /* convert from 4 digit year */
+               else if (year < 100) {
+                       /* Convert to tm_year assuming current century */
+                       year += (tm->tm_year / 100) * 100;
+
+                       if (year == tm->tm_year - 1)
+                               year++;         /* Common off by one error */
+                       else if (year < tm->tm_year)
+                               year += 100;    /* must be in next century */
+               }
       }

       if (year < 0 &&